Drilling ships and semi-submersible platforms are equipped with drilling rigs to support the necessary lifting of equipment under various offshore operations. Usually, these derricks are equipped with passive and active compensator systems to compensate for relative movements between the vessel and the seabed due to vessel wave impact.
The wave compensation systems are designed for handling of maximum loads at the seabed or down hole. This is often accomplished by installing a passive compensator in the top of the drilling rig. This passive compensator compensates the weight hanging on the crown block relative to the movement between the vessel and the seabed by means of a cylinder system of a pre-charged accumulator. Other systems are compensated by active draw-works/winches without any passive/autonomous compensator systems.
The traditional draw-work system lifts also only include one single wire that is led through many discs to achieve the correct ratio between the crown block and the lift block. This could result in challenges related to security, the lifetime of the cable, and noises due to the high speed of the cable and in the winch system.
The passive compensation system in the top of drilling rig is heavy and not easily accessible for maintenance. Considerable weight in the top of the derrick is detrimental to the properties of the vessel. They also have a large cylinder area adapted to the need to compensate passively for maximum load at ultra deep water operations. This large cylinder area results in large load variations at small loads at shallower waters and easier well intervention operations. In case of an active draw-work-configuration, the rig has no back-up in case of possible loss of electrical power supply and is therefore less suitable for critical operations.
The drilling rigs (derricks) consist traditionally of a pyramidal steel framework with square or rectangular cross-section assembled together to a fixed structure. The drilling rigs have a complex structure. They comprise multiple parts that may lead to high risk of falling objects. The height of the rigs is fixed without any possibilities to change the height when the drilling rigs are mounted together. This makes the maintenance work on top of the drilling rig more difficult.
Ice formation on the framework of the drilling rigs is another challenge of drilling rigs according to prior art. This is especially a problem when operating in arctic environments.